Recording capacity of a single layer of an optical disc depends to a large extent on wavelength of a semiconductor laser used and a number of apertures (NA) of an objective lens. The shorter the wavelength of semiconductor laser is or the larger the number of apertures (NA) is, the larger recording density can be made and thereby the more capacity of a single layer can be increased. As the principal optical disc drive, a DVD (Digital Versatile Disc) drive using the red-color light in the wavelength near 650 nm and an objective lens having NA of 0.6 is available in the present market. However, an optical disc drive using a semiconductor laser of blue-purple color of the wavelength near 405 nm as a light source and an objective lens having NA of 0.85 is started to be delivered as the new optical disc drive having the recording density higher than that of DVD. When it is considered to use a shorter wavelength in order to improve recording density in future, development of such semiconductor laser source having the wavelength shorter than that of such blue-purple color will naturally be accompanied with considerable difficulty, because wavelength is in the outside of ultraviolet domain. Moreover, since a limit value of NA of the objective lens in the air is 1 (one), realization of improvement in the recording density by an objective lens is considered to be difficult considerably.
Under the situation explained above, a double-layer structure of a sheet of optical disc has been implemented as a system to increase storage capacity of the optical disc. Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 956-960 proposes a technology of phase variation disc of double-layer structure. When a laser beam is radiated to an optical disc of double-layer structure, a problem of cross-talk between layers occurs here, because adjacent layers are irradiated simultaneously with the laser beam. In order to suppress this problem, a large interval is provided between layers. Since the laser beam is focused at the target layer and is also deviated from the focal point thereof at the layers other than the target layer (relevant layer), cross-talk can be lowered.
Meanwhile, when a layer-to-layer interval is widened, spherical aberration is generated as a problem. A recording layer is embedded within a polycarbonate material having refractive index different from that of the air and spherical aberration is different depending on depth from disc surface. An objective lens is designed to make small its spherical aberration for the particular layer. Therefore, spherical aberration is generated when the focal point of laser beam is shifted to the other layers. This aberration can usually be compensated by setting an expander lens optical system formed of a couple of lenses or a liquid crystal element at the location before the objective lens. Namely, aberration can be compensated by varying distance between a couple of lenses or phase of the liquid crystal element. However, a large spherical aberration cannot be compensated, considering that compensation range of liquid crystal element or moving mechanism of lens is realized within a small-sized optical disc drive. Accordingly, it may be thought substantially difficult to use actually an optical drive for a multilayer optical disc including a sufficiently wide layer-to-layer interval. As a result, a multilayer optical disc having a narrow layer-to-layer interval has to be used, resulting in a problem of layer-to-layer crosstalk.
According to JP-A No. 2005-3602084, to reduce crosstalk explained above, it is used that when the reflected laser beam from a multilayer optical disc is focused, focal positions of the reflected laser beams from the target layer and adjacent layers thereof are different on an optical axis. Only the target reflected laser beam can be extracted by arranging a fine mirror on this optical axis and thereby crosstalk can be reduced. However, since the reflected laser beam from an optical disc is bent in the lateral direction for the optical axis in this system, an optical pickup is inevitably increased in size. Moreover, JP-A No. 2002-367211 proposes a method to remove the reflected laser beams from the adjacent layers by utilizing a critical angle prism. In this method, the reflected laser beam from the relevant layer is changed to a collimated parallel laser beam but it is implemented to remove the laser beam having an inclination angle larger than the predetermined angle for the optical axis with the critical angle prism by utilizing that the fact that the reflected laser beams from adjacent layers become the divergent laser beam or convergent laser beam. In this system, an optical pickup is inevitably increased in size, because a couple of critical angle prisms are used.